Off goes defender Joel Ward, on comes attacker Bakary Sako. Palace smell blood.
Palace threaten again. This time it is Zaha, weaving into space in the box before firing an effort that Joe Hart pushes away.
Oh, so close to 2-2! Yohan Cabaye fires a shot from 20 yards that has Hart beaten but comes back off the post. The home side have their tails up.
Palace a different side since the break.
Luka Milivojevic makes no mistake, sending Hart the wrong way. Now we have a game on our hands.Copyright: PA
Now then... huge chance for Palace to get back into this game.
Townsend goes down under a challenge from Angelo Ogbonna and the ref points to the spot.
Palace threaten early in the half as Andros Townsend's cross is cleared only as far as Luka Milivojevic. His shot almost falls to Yohan Cabaye but he can't flick the ball towards goal.
West Ham have made a change at the break. Arthur Masuaku is on for Aaron Cresswell. You would assume that is down to an injury but I saw no evidence of it during the first half.
It is pretty clear from that first half which of these two sides came into the game following a superb comeback victory midweek and which took a hammering.
West Ham have been ruthless - a spring in their step emerging after Hernandez's opener.
Palace were floored by going 1-0 down. They haven't recovered.
The half ends with a chance for Palace, which comes from a corner and sees Loftus-Cheek head wide.
Plenty for Roy Hodgson to consider and correct.
It is all going to plan for West Ham.
James Tomkins takes no prisoners. He smashes the ankle of Andre Ayew and leaves the striker needing treatment. Somehow the Palace defender escapes a booking.
Oh my word! Andre Ayew is on fire!
When the West Ham striker picks up the ball on the break he still has plenty to do, but do it he does. He weaves forward, turning Scott Dann inside out before lashing a shot from 20 yards in off the bar.
That is three in two games for the forward after his brace at Wembley.
This is the best of the lot, though. Worth recording Match of the Day for alone.Copyright: PA
Andros Townsend hasn't got into this game yet but he swings over a decent cross that Joe Hart does well to come out and claim.
Fosu-Mensah is back up and looks ready to continue. Meanwhile, Loftus-Cheek heads over from the free-kick.
Pablo Zabaleta into the book for a late, diving challenge on Timonthy Fosu-Mensah. That is his fifth yellow of the season, which means he misses the Liverpool game.
Fosu-Mensah is in pain. Would be very bad luck for Palace to lose a man who only came on for the injured Van Aanholt earlier in the half.
Amazing double save!
James Tomkins' header from close range is palmed away by the diving Joe Hart, who gets up sharpish to somehow turn Jeffrey Schlupp's follow-up over the bar with his forearm. Don't think he knew much about the second save, but it preserves his side's lead.
The West Ham fans in fine voice now. "How *ahem* must you be, we're winning away" is the chant.
The Palace Twitter account might want to keep looking elsewhere. West Ham lead!
It is a cracking little finish from Javier Hernandez, who is perfectly placed to slot home with the outside of his boot from Aaron Cresswell's low ball.
It is the Hammers first attack, on the break after Wilfried Zaha felt he should have had a penalty following a tangle with Jose Fonte at the other end.Copyright: Reuters
Even the Palace Twitter account is turning attention elsewhere...
Jeffrey Schlupp gets in down the left, just ahead of his marker but his cross - designed for the middle of the box - finds the 15th row back behind the goal.
Free-kick after free-kick after free-kick. Neither team able to lock down possession for any period of time. It's dire.